1. Field of the Invention
The present invention relates to a fish finder that transmits and receives ultrasonic waves to find a shoal of fish in the water, and, more particularly to a fish finder that detects occurrence of interference caused by other ultrasonic devices and automatically performs interference removal.
2. Description of the Related Art
In general, a fish finder mounted on a fishing boat has a problem of interference caused by ultrasonic waves transmitted by other ultrasonic devices mounted on the fishing boat or other boats operating in sea areas near the fishing boat.
Conventionally, as means for solving this problem, a fish finder having an interference removal function is known. With this interference removal function, the fish finder compares, for each identical depth, reception signals obtained by transmission and reception performed plural times and selects a weakest signal to perform processing for interference removal.
When the interference removal function is used, there is an inconvenience that not only interference signals but also useful echoes including information on a shoal of fish or the like is controlled. Thus, when no interference occurs, the interference removal function should be turned off as much as possible. However, in the conventional apparatus, a user needs to judge at any time whether there exists interference and manually perform on/off operation for the interference removal function. As a result, the user is forced to perform troublesome operation.